The present invention relates to a phase retarder comprising at least two optically anisotropic layers, and a circularly polarizing plate equipped with the phase retarder. In detail, the invention relates to a phase retarder suitable for a quarter wave plate used in a liquid crystal display of reflection type, a GH-LCD, a PS conversion devise, an anti-reflection film or a pick-up for writing on optical discs.
Various quarter wave plates have been proposed and used in optical devices such as an LCD of reflection type, a pick-up for optical discs and a PS conversion devise. However, most of the conventional plates can function as a quarter wave plate merely at a specific wavelength.
Each of Japanese Patent Provisional Publication Nos. 10(1998)-68816 and 10(1998)-90521 discloses a phase retarder comprising two optically anisotropic polymer films. The phase retarder of 10(1998)-68816 comprises a quarter wave plate (which retards birefringent light by xcex/4) and a half wave plate (which retards birefringent light by xcex/2) laminated so that the optical axes may be crossed. The phase retarder of 10(1998)-90521, on the other hand, comprises at least two retarders having a retardation value of 160 to 320 nm, and the retarders are laminated so that their slow axes may be neither parallel nor perpendicular to each other. Each disclosed phase retarder comprises two polymer films, and gives the phase retardation of xcex/4 within a wide wavelength region.
The phase retarders disclosed in Japanese Patent Provisional Publication Nos. 10(1998)-68816 and 10(1998)-90521 are considerably thick, because they are prepared by laminating two polymer films. In examples described in 10(1998)-90521, two or more polycarbonate films (Sumika right SEF-460270B and SEF-460266B7, Sumitomo Chemicals Co., Ltd.) are laminated. Since each polycarbonate film has a 60 xcexcm thickness, the total thickness is 120 xcexcm or more. Also in 10(1998)-68816, though there is no description of the thickness, the total thickness is presumed to be 120 xcexcm or more because polycarbonate films are also used in example embodiments.
A quarter wave plate or a circularly polarizing plate is mainly used in a liquid crystal display, which is relatively thin and light as compared with other image displaying devises. Accordingly, the quarter wave plate or the circularly polarizing plate is preferred to be as thin and light as possible. In addition, a liquid crystal display (e.g., a liquid crystal display of host-guest type) in which a quarter wave plate is provided between two substrates of the liquid crystal cell has been recently proposed. The quarter wave plate used in that display is particularly required to be thinner than a conventional one because the plate is placed inside the liquid crystal cell. Even if it is placed outside the cell, a thin quarter wave plate gives very good effects when used in a liquid crystal display which adopts plastic substrates to make the display thin and light or in a liquid crystal display of reflection type which is developed to reduce parallax.
An object of the invention is to provide a very thin quarter wave plate usable within a wide wavelength region.
The present invention provides a phase retarder which comprises an optically anisotropic layer A essentially giving the phase retardation of xcfx80 at wavelength of 550 nm and an optically anisotropic layer B essentially giving the phase retardation of xcfx80/2 at wavelength of 550 nm, said layers A and B being laminated so that the retarder may have such an optical characteristic that a retardation value per wavelength is within the range of 0.2 to 0.3 at each wavelength of 450 nm, 550 nm and 650 nm, wherein the total thickness of the optically anisotropic layers A and B is within the range of 500 nm to 20 xcexcm.
The invention also provides a circularly polarizing plate which comprises a phase retarder and a polarizing membrane provided thereon, said phase retarder comprising an optically anisotropic layer A essentially giving the phase retardation of xcfx80 at wavelength of 550 nm and an optically anisotropic layer B essentially giving the phase retardation of xcfx80/2 at wavelength of 550 nm, said layers A and B being laminated so that the retarder may have such an optical characteristic that a retardation value per wavelength is within the range of 0.2 to 0.3 at each wavelength of 450 nm, 550 nm and 650 nm, wherein the total thickness of the optically anisotropic layers A and B is within the range of 500 nm to 20 xcexcm.
The applicants have succeeded in preparing a very thin quarter wave plate usable within a wide wavelength region. The plate comprises optically anisotropic layers A and B, and the total thickness of the layers is less than 10 xcexcm. In detail, the phase retarder is prepared by forming at least one (preferably, both) of the optically anisotropic layers A and B from liquid crystal molecules and then transferring them. Thus, a thin and light quarter wave plate or circularly polarizing plate suitable for a liquid crystal display and usable within a wide wavelength region can be obtained.
Since the optically anisotropic layer is formed from liquid crystal molecules, the optical characteristics can be easily controlled. The optical direction of the optically anisotropic layer containing the liquid crystal molecules can be also easily controlled by selecting the rubbing direction of the liquid crystal molecules. Consequently, it is unnecessary to cut films into chips in a conventional manner.
Further, the optically anisotropic layer formed from liquid crystal molecules has strong heat-resistance as compared with the layer of polymer film. When a quarter wave plate comprising the optically anisotropic layer of polymer film is placed between the two substrates of the liquid crystal cell, it is feared that the polymer film may be transformed at the temperature for aligning rod-like liquid crystal molecules in the cell.
According to the present invention, a very thin quarter wave plate usable within a wide wavelength region can be easily prepared.